Circuit arrangement for controlling a colour television display tube



Jan. 28, 1964 'Filed Dec. 16, 1960 INTEG AI'OR J. DAVIDSE ETAL CIRCUITARRANGEMENT FOR CONTROLLING A COLOUR TELEVISION DISPLAY TUBE SOURCE OFINTEGRATOR ADDER GATE 3 SheetsSheet 2 AMPLIFIER GATE OSCILLATOR TRIGGERDELAY NETWORK PHA s 5 DE TEC TOR VARIABLE REACTA NCE INVENTOR JANDAVIDSE BERNAROUS +1.1. CORNELISSEN AGE Jan. 28, 1964 J DAVlDsE ETAL3,119,898

CIRCUIT ARRANGEMENT FOR CONTROLLING A COLOUR TELEVISION DISPLAY TUBEFiled Dec. 16, 1960 3 Sheets-Sheet 3 parse-ran 45 "(5/ F I 6 5OSCILLATOR CONTINUOUS 21 ucmL T I 1VARIABLE I. REAcTANcE AGE UnitedStates Patent 3,119,393 CIRCUIT GEMENT FUR CONTROLLING A COLUURTELEVHSEQN DESPLAY TUBE Jan Davidse and Bernartius l lenricus lozet.Eornelissen, Eindhovten, Netherlands, assignors to North AmericanPhilips Company, Inc, New York, N.Y., a corporation of Delaware FiledDec. 16, 196%, Ser. No. 76,345 Claims priority, application NetherlandsJan. 20, 196i) 5 Claims. (Cl. 1785.4)

This invention relates to a circuit arrangement for controlling a colourtelevision display tube comprising a display screen with colour stripsand index strips and a control-electrode for controlling an electronbeam produced in the tube, this beam being deflected across the screenby means of signals obtained from horizontal and vertical deflectiongenerators with the aid of deflectors, the index strips emittingsecondary electrons when they are struck by the beam, which secondaryelectrons produce an index signal which is fed, after being added to thetelevision signal to be reproduced, to the controlelectrode of the tube.

The invention also relates to a circuit arrangement for controlling acolour television display tube comprising a display screen with colourstrips and index strips and two control-electrodes, in which tube asignal fed to the first control-electrode is capable of controlling apilot beam and a signal fed to the second control-electrode is capableof controlling a writing beam, the signal controlling the pilot beamhaving a considerably higher frequency than the signal controlling thewriting beam, these beams being deflected across the screen by means ofsignals obtained from horizontal and vertical deflecting-generators withthe aid of deflectors, an index signal being formed by filtering out thesignal which is developed when the index strips are struck by the pilotbeam, these strips thus emitting secondary electrons, which indexsignal, after being added to the colour television signal to bereproduced, is fed to the control-electrode for controlling the writingbeam.

Circuit arrangements comprising index tubes (apple tubes) are used incolour television receivers and are described inter alia in I. R. E.Conventional Record 1956, Part 3, Electron Devices and Receivers, pages9499. From this \article it is evident that difiiculties arise from thedifference in transit time of the secondary electrons emanating fromindex strips at the edges and at the centre of the screen, whichelectrons are emitted when the index strips are struck by the scanningelectron beam. The difference in transit time produces a phase shift ofthe final index signal so that, if no special precautions are taken, theresulting index signal does not provide a correct information about theinstantaneous impact point of the beam on the screen with respect to thecolour strips.

In the said article it is stated (page 98, left-hand column, tourthparagraph) that this difficulty may be obviated by providing a patternof the index strips diflering from that of the colour strips, thedifference varying with the place on the screen, the diflerence intransit time of the secondary electrons being thus compensated.

It will be obvious that this measure is costly and critical, since theapplication of index strips with a varying pattern is to be carried outwith utmost accuracy, which involves a costly production, whilst evensmall errors in the pattern variations render the transit-tirnecompensation less effective.

The circuit arrangement according to the invention is based on quitedifferent principles to correct the said errors in the transit time andis characterized in that in order to correct the phase error in theindex signal produced owing to the diflerence in transit time of thesec- 3,l 1 .93% Patented Jan. 2%, 1964 ice iond'ary electrons emanatingfrom index strips at the edges and at the centre of the screen anopposite phase error is introduced into the signal fed to thecontrol-electrode by means of a phase-shifting member included in thearrangement, to which member correction signals are fed which arederived from signals obtained from the horizontal and verticaldeflecting-generators.

In a circuit arrangement in which use is made of an index tube havingtwo beams, the same idea of the invention may be carried out and thisarrangement is characterized in that in order to correct the phase errorin the index signal produced owing to the diflierence in transit time ofthe secondary electrons emanating from index strips at the edges and atthe centre of the screen and/ or owing to turning of the two beams abouteach other during the deflection an opposite phase error is introducedeither into the signal ted to the electrode for controlling the pilotbeam or into the signal fed to the electrode for controlling the writingbeam, to which end either in that part of the arrangement which producesthe signal controlling the pilot beam or in that part of the arrangementin which the index signal is worked up, a phase-shifting member isprovided to which correction signals are fed, which are derived fromsignals obtained from the horiaontal and vertical deflecting-generators.

It should be noted for the use of an index tube with two beams thedisadvantage of the phase error in the index signal owing to the turn ofthe two beams about each other during the deflection is described in anarticle in LRE. National Conventional Record: 1957, Part 3: Broadcastand Television Receivers, Electron Devices, pages 238242, particularlypage 240, left-hand column, last line to right-hand column, fourthparagraph. From this article it is furthermore evident that thedisadvantage of the last-mentioned phase error may be obviated byproviding a curvature of the index strips which is differout on the topside and on the lower side of the display screen from the curvature ofthe colour strips. It will be obvious that also this method renders theproduction more expensive.

There is, moreover, the disadvantage that, if the said phase errors arecompensated by means of the geometry of the index strips, a fluctuationof the mains voltages may render this compensation either excessive orinsufficient.

In the circuit arrangement according tothe invention this disadvantagedoes not occur, since the correction sig nals fed to the phase-shiftingmember are either automatically or manually variable, if this should berequired with a view to mains voltage fluctuations to obtain the correctphase correct-ion.

A few embodiments of circuit arrangements according to the inventionwill now be described more fully by way of example, with reference tothe accompanying drawings, in which:

FIGURE 1 shows a first embodiment comprising an index tube with twoelectron beams, in which the phase of the signal controlling the pilotbeam is varied by a direct variation of the phase of the pilot signal;this figure illustrates, moreover, the possibility of varying the phaseof the signal derived from the incoming reference signal before :it isuse-d in working up the index signal.

FIGURE 2 shows a second embodiment also comprising an index tube withtwo beams, in which the phase of the pilot signal is indirectly varied.

FIGURE 3 shows an embodiment comprising an index tube with one beam.

FIGURE 4 shows a phase-shifting member which may be used in the circuitarrangement according to the invention.

FIGURE 5 serves to clarify FIGURE 4.

FIGURE 6 shows a second possible embodiment of a phase-shifting memberfor use in the circuit arrangements, and

FIGURE 7 shows a detail of the phase-shifting member of FIGURE 6.

Referring to FIGURE 1, reference numeral 1 designates an index tubecomprising a display screen 2, a collector 3, a first control-electrode4, a second control-electrode and a cathode 6. The display screen 2 isprovided in known manner with colour strips and interconnected indexstrips which are brought outside the tube 1 via the conductor 7. Thecollector 3 is also brought outside the tube 1 via the conductor 8 andconnected via this conductor to a direct-voltage source supplying a highpositive direct voltage. Between the conductors 7 and 8 provision ismade of a resistor 9, across which the index signal is produced, whichis transferred via the sideband amplifier 10, which filters out thedesired sideband frequency, to the mixing circuit M Thecontrol-electrode 4 serves to control the so-called pilot beam and inthe present embodiment this control-electrode receives a signal having afrequency of 36 mc./s. The second control-electrode 5 serves to controlthe so-called writing beam and this control-electrode receives theworked up index signal, which contains, moreover the luminanceinformation and the colour information required to reproduce the colourtelevision image on the display screen 2 by means of the colour strips.The cathode 6 emits electrons to produce the pilot beam and the writingbeam with the aid of beam-screening electrodes. The signal to be fed tothe control-electrode 5 is obtained by means of a local oscillator 1 1producing a signal of a frequency of 36 mc./s., which signal is fed to afirst mixing stage M To this mixing stage M is also fed the coloursignal, which is detected elsewhere in the receiver and of which themodulation frequencies are lying around a frequency of about 4.5 mc./s.After mixing a signal is obtained of which the modulation frequenciesare lying around a frequency of about 40.5 mc./s. and which is fed to asecond mixing stage M to which is, moreover, fed the signal of 4.5mc./s. derived from the burst signal. After mixing of the signals fed tothe mixing stage M a modulated signal is obtained which has a carrierfrequency of 36 mc./s. and which contains the colour information and,moreover, the information supplied by the burst signal. This signal isthen fed to the mixing valve M in which it is mixed with the indexsignal of, in this case, 43 mc./s. or 29 mc./s., which depends upon thefrequency of the pilot signal, upon the number of index strips and uponthe velocity with which these index strips are scanned. After mixing amodulated signal is obtained which has a carrier frequency of 7 mc./s.,to which the luminance signal Y is added in the adder 12, after whichthe whole signal is fed to the control-electrode 5.

As stated above, also the signal obtained from the oscillator 11 of 36mc./ s. is fed to the control-electrode 4 so that the pilot beam iscontrolled in the rhythm of this frequency, secondary electrons beingformed a the impact of this pilot beam on the screen, which electronsWander from the display screen 2 to the collector 3. These electrons arecomparatively slow and since the distances from the edges of the displayscreen 2 to the collector 3 are considerably smaller than the distanmsfrom the centre of the screen to this collector, the time required forthe electrons from the edges of the screen to reach the collector isconsiderably shorter than the time required for the electrons from thecentre of the screen. Thus the index signal of the sideband frequenciesof about 43 mc./s. and 29 mc./s., produced across the resistor 9, doesnot provide a true information about the instantaneous impact point ofthe pilot beam on the screen with respect to the colour strips. Phaseerrors are introduced into the index signal, which errors give rise tocolour errors in the reproduced image, of no precautions to eliminatethe transit-time enrors are taken.

It should be noted that in the embodiment shown in FIGURE 1 the sidebandfrequencies of 43 mc./s are filtered out. Of course, as an alternativethe sideband frequencies of 29 mc./s. could be filtered out.

In accordance with the invention the said phase error can be eliminatedby introducing an opposite phase error into the signal controlling thepilot beam or into the signal controlling the writing beam. If it isintnoduced into the signal controlling the pilot beam, the phase errorof the index signal is eliminated by the introduction of this oppositephase. If the opposite phase is introduced, however, into the signalcontrolling the writing beam, the colour enrors occurring in the absenceof the said compensation are directly compensated.

In order to correct the signal controlling the pilot beam, the circuitarrangement shown in FIGURE 1 may be provided with a phase-shiftingmember 13, to the input terminal of which is fed a signal obtained froman adder 114. In this adder 114 the signals emanating from integrators15 and 16 are added. To the integrator 15 is fed the sawtooth signal 17from the horizontal deflection-generator and to the integrator 16 is fedthe sawtooth signal 18 from the vertical defiection-generator.Consequently, the output of the integrator 15 has produced across it aparabolic signal of which the symmetry axis must lie each time half aperiod after the beginning of a period of the sawtooth signal 17, sincethe deviations owing to the transit-time errors are at a minimum at theside edges of the display screen, so that the compensation in the centreof the screen is to be at a maximum. The parabolic voltage 19 maytherefore have a waveform as shown in FIGURE 1. The integrator 15 may beformed, for example, by the series combination of a resistor and acapacitor, the RC-time of this network having to be high with respect toone period of the sawtooth signal 17. The output signal is to beobtained from the capacitor. If a further improved integration isdesired, a discharge valve connected as a Miller integrator may beemployed. The signal 17 is then fed to the control-grid of the dischargevalve and the output signal may be derived from the anode. In the lattercase the output signal is to be derived via a blocking capacitor inorder to avoid undue direct-voltage components in the output signal.

The same applies to the output voltage 20 of the inte grator 16. Theoutput of this integrator has to supply also an approximately parabolicvoltage of image frequency in order to compensate the transit-timeerrors which are at a minimum on the lower and the upper side of thescreen and at a maximum at the centre of the screen. The integrator 16may be constructed similarly to the integrator 15; it should only beconsidered that the image frequency is materially lower than the linefrequency and that the RC-time of the integrator 16 is to be adaptedthereto.

In order to correct the signal controlling the writing beam, two waysmay be taken in principle.

Firstly, the index signal emanating from the sideband amplifier 10 canbe shifted in phase. This involves, however, great difliculties, sinceowing tothe non-linearity of the deflection signals the frequency of theindex signal may vary so that a phase-shifting member included after theamplifier 10 is to be suitable for shifting the phase of the signalwhich may have more than one frequency. This can be carried out onlywith difiiculty. It is therefore preferable to choose the second way andto shift the phase of the signal derived from the burst signal appliedto the mixing stage M and having always a fixed frequency of 4.5 mc./s.To this end the block 21 is provided, which is shown in FIGURE 1 bybroken lines, the incoming burst signal 22 being fed thereto, whichsignal 22 is converted in 21 into a continuous signal of 4.5 mc./s.,.

which can be mixed with the signal containing the colour information, ofwhich the modulation frequencies are lying around a frequency of about40.5 nic./s.- This conversion may be carried out in various ways, ofwhich one will be described more fully hereinafter.

The device 21 is, moreover, capable of shifting the phase of the derivedsignal of 4.5 mc./s. and to this end the output voltage of the adder 14'is fed to the device 21. The devices 14', 15 and 16' are similar to thedevices 14, 15 and 16 and shown only in broken lines, whilst it isindicated that they may be employed to control a phase-shifting member13 and a device 21, if one of them is employed in the arrangement ofFIGURE 1.

A second embodiment is shown in FIGURE 2. In this figure, in whichcorresponding parts are designated as far as possible similarly to thosein FIGURE 1, the signal of 4.5 mc./s. derived from the burst signal isnot used for working up the index signal but it is med with the signalof 36 mc./s. from the local oscillator 11 to obtain a signal of 40.5mc./s., which is fed to the electrode 4 controlling the pilot beam. In asimilar manner as in FIGURE 1 the signal of 4.5 inc/s. derived from theburst signal 22 may be used with the aid of the device 21 to shift itsphase to obtain the required phase correction.

It should furthermore be noted that the signal derived from the burstsignal must not be used again in that part of the arrangement in whichthe index signal is Worked up, since the reference required for thecolour signal to be reproduced is introduced via the pilot beam into theindex signal. It is therefore sufiicient to mix the signal of 36 mos.from the local oscillator 11 in the mixing stage M with the coloursignal, after which a signal is obtained which contains the requiredcolour information and of which the modulation frequencies are lyingaround a frequency of about 40.5 'mc./s. The latter signal is mixed inthe mixing stage M with the index signal from the sideband amplifierit), which signal has a frequency of 47.5 rnc./s. in this embodiment andcontains, moreover, the required reference for a colour image to becorrectly reproduced, so that a signal of 7 mc/s. is produced, which isted to the control-electrode 5 after the luminance signal Y has beenadded thereto in the adder 12..

A third embodiment is shown in FIGURE 3. The index tube shown in thisfigure is driven by a single electron beam and may be connected in themanner described in copending US Patent application Serial No. 29,268,filed May 16, 1960; by means of gate circuits it is provided that novideo information is introduced into the index signal, when the electronbeam scans an index strip.

In order to provide a clearer insight in this controlmethod, it will nowbe described briefly. fie electrons emitted by the cathode 6 arecontrolled with the aid of the single controiclectrode 23 and afterstriking the display screen 2; they serve both for the reproduction ofthe colour television signal and for the production of the indexsignals. The index signal of about 7 mc./s. is amplified by the sidebandamplifier 1t and then fed to a gate circuit 24, which is each timeopened at an instant shortly before an index strip is struck by theelectron beam and is closed at the instant shortly after an index stripis struck by the electron beam. The index signal thus free of videoinformation is fed to the trigger circuit 25, the output of which hasproduced across it pulses having the frequency and the phase of theindex signal from the electrode 7, which pulses are fed to the phasedetector 2.6. In the phase detector 26 the signal from 25 is comparedwith the signal from a local oscillator 27, so that in the case ofnon-synchronism of the two signals a direct voltage is produced at theoutput of 26, by which voltage the oscillator 27 can be readjusted bymeans of the reactmce circuit 28 until synchronism between the signalsfrom 25 and 26 is attained. The output signals of 27 are fed as gatepulses to the gate circuit 24, which is thus opened each time at thecorrect instant. The gate circuit 24 is, moreover, opened for a shortinstant, by means of pulses of line frequency fed via the conductor 629, at the beginning of each line, so that the oscillator 27, which hasgot out of synchronism during the preceding line fly-back period can bebrought to the correct rhythm.

The signal [from 27 is fed via the delay network 30 to a second gatecircuit 31, Which brings the controlelectrode 23 each time to a fixedpotential so that at these instants no video information can occur inthe electron beam. The delay time of the network 34? corresponds to thetransit time of the electrons from the control-electrode 23 to thedisplay screen 2, so that the electron beam does not contain videoinformation, when striking an index strip. To the gate circuit 31 arefed via the conductor 32 also line pulses to bring the said electrode 23to the fixed potential each time at the beginning of a line.

As stated above, the oscillator 27 is in synchronism with the indexsignal, which may thus be used, in addition, to Work up the videoinformation. To this end the signal delayed in 3th is fed via theconductor 33 to the mixing stage M To this mixing stage is fed via theconductor 34 the colour information, which is modulated around afrequency of 4.5 mc./ s. At the output of the mixing stage M a signal isthus obtained, which contains the colour information modulated around asignal of 1-1.5 mc./s.

The latter signal is fed to the mixing stage M in which it is mixed withthe signal derived from the burst signal 22. The signal produced at theoutput of M of 7 mc./s. contains therefore the information of the indexsignal, of the burst signal and of the colour and in the adder 12 theluminance signal Y is added thereto, after which the whole signal is fedto the control-electrode 23 via the gate circuit 31. The gate circuit 31is keyed by means of the pulses from 3t so that it passes normally thesignal from 12 during the periods in which the electron beam scans thecolour strips of the screen 2, which is not the case during the scanningof the index strips, since then the electrode 23 is brought to the saidfixed potential with the aid of the gate circuit 31.

The phase shift required in accordance with the invention is obtained inthe manner described with reference to FIGURES l and 2 by shifting thephase of the signal of 4.5 mc./s. derived from the burst signal 22 inthe device 21. To this end the correction signals from 14 are fed to thedevice 21.

A possible embodiment of the phase-shifting member 13 shown in FlIGURE lis illustrated in FIGURE 4.

The phasesshitting member 13 in bridge arrangement consists of acentral-tapped coil 36 and a series combination of a resistor 39 and areactance element formed in the present embodiment by the dischargevalve 37, connected as a capacitative reactance element, thiscombination being connected in parallel with the coil. The dischargevalve 37 is connected by its cathode to the lower end of the coil 36 andvia the separation capacitor 38 to the resistor 39. The anode of thevalve 37 is furtherin re connected 'via the capacitor 40 to the controlsgrid, which, in turn, is connected via the resistor 41 to thecathode. Consequently, the elements 37, 40 and 41 constitute, in knownmanner, a variable capacity, of which the variation can be produced bythe signals fed to the control-grid. These signals are the correctionsignals from the adder 14, which signals are fed to the saidcontrol-grid by way of the separation resistor 42 and the parallelcombination 43.

The coil 36 constitutes one branch of the bridge, the series combinationof the resistor 39 and of the elements 37, 40 and 41, connected as acapacity, constitutes the other branch. The input voltage of 36 mc./s.from the local oscillator 11 is fed to the bridge circuit via the inputterminals 44 and 45 and the output voltage can be derived via theamplifying valve 35 of FIGURE 4 from the output terminals 46 and 47.

The phase shift of the non-amplified output voltage V between thecentral tapping of the coil 36 and the junction of the resistor 39 andthe blocking capacitor 38 with respect to the voltage across the coil 36may be accounted for by means of the vector diagram of FIGURE 5.

The part between the said central tapping and the resister 39constitutes a first alternatingwoltage source and the part between thistapping and the cathode of the valve 37 constitutes a secondalternating-voltage source. The sum of the voltages supplied by thosetwo sources is operative across the series combination of the resistorand the capacity.

The voltage across the resistor may be considered to be a vector V whichis at an angle of 90 to the vector 5 of the voltage across the capacity.Their sum V +V must be equal to the sum 7 4 1 of the voltages,considered as the vectors V and V produced by the voltage sources.

The vector diagram thus obtained constitutes a rectangular triangle, ofwhich V +T is the base and V and V are the right-angle sides. It isknown that a circle of which 7 4-1 is the diameter goes through the apexof the right-angle triangle and with a variation of the capacity thesize of the vectors V and V will change, it is true, but the apex of thetriangle will move along the circle circumference.

The output voltage V may also be considered as a vector V of which oneend lies on the circumference and the other end lies on the centre ofthe circle. Consequently, when the capacity is varied, the vector Vturns, whilst its size remains constant, but the angle thereof to thevectors V and 7 will vary. Since the said angle is the phase angle ofthe output voltage V with respect to the voltage across the coil 36, thephase shift occurring is accounted for.

It will be obvious that the valve 37 may be connected, as analternative, as an inductive reactance element; moreover, any othervariable reactance element may be employed, for example, a junctiondiode, controlled in the blocking direction and similar elements.

The resistor 39 may also be variable and also the phase bridge itselfmay be constructed in various known ways.

In order to minimize the influence of the capacity variations on thebridge, the impedance of the coil 36 is high for the frequency of 36mc./s. with respect to the impedance of the series combination of theresistor and the capacitor.

The output terminal 47 is connected to earth and the output terminal 46leads to the control-electrode 4 of the index tube 1 shown in FIGURE 1.

Although the phase-shifting member 13 is described for the case in whicha signal of 36 mc./s. is supplied thereto, it will be obvious that alsoa signal of 4.5 mc./s. derived from the burst signal may be shifted inphase in a similar phase-shifting member after adaptation. In this casea part of the device 21 consists of the phaseshifting member shown inFIGURE 4 and the further part of the device 21 consists of anarrangement which converts, in known manner, the burst signal occurringonly at the back porches of the line-synchronizing pulses into acontinuous signal.

It should be noted that the circuit 43 serves to avoid a react-ion ofthe oscillator signal to the adder 14. If a signal of 36 mc./s. is fedto the terminals 44 and 45. this circuit is tuned to 36 mc./ s. and ifthe signal derived from the burst signal is supplied thereto, thecircuit is to be tuned to 4.5 mc./s.

The device 21 may, however, also be constructed as is shown in FIGURE 6.In this case the device 21 consists of a local oscillator 48, whichproduces a sinusoidal signal of about 4.5 mc./s. This local oscillatorhas to produce a signal which is synchronous with the burst signal 22occurring during the back porches of the line-synchronizing pulses. Tothis end the burst signal 22 is compared in the phase detector 49 withthe oscillator signal from 48, after which the output voltage of 49 isfed to the reactance circuit 59, by which the oscillator 48 can bereadjusted. In principle, it would be sutficient to feed directlycorrection signals from 14 to the reactance circuit St in order toobtain in this way the required phase shift of the final signal of 4.5rnc./s.

However, in this case very high correction signals are required to bringabout the desired phase shift in the output signal of 43, since thecontrol-loop formed by the devices 49, 5s and 48 tends to equalize thephase of the oscillator signal to the phase of the burst signal 22.

The arrangement shown in FIGURE 6 is based on the recognition of thefact that the burst signals 22 occur only during the back porches of theline-synchronizing signal so that during the remaining part of each lineperiod the said loop is left to itself. Consequently, the required phaseshift can be obtained with comparatively :small amplitudes of thecorrection signal by feeding the correction signal during the saidremaining part of the period via a gate circuit 51 to the reactancecircuit 50. This gate circuit is keyed to this end by the fly-backpulses 52 from the line-deflection generator.

The fly-back pulses have a duration which corresponds to the horizontalfly-back time, so that the correction signals from 14 will only beoperative during the onward stroke of the horizontal deflection, whilstthe loop 48, 49 and 56 can bring the oscillator 48 into synchronismduring part of the horizontal fly-back time with the aid of the burstsignals 22 occurring at the back porches.

In the aforesaid manner it is therefore possible to obtain a continuoussignal of 4.5 mc./s. from the device 21, of which signal the phase isshifted during the horizontal onward stroke, i.e. just during the timein which a colour television signal is to be reproduced.

It is also possible to use key pulses 52, which have a much shorterduration and which occur only during the occurrence of the burst signalsat the said back porches. However, in this case separate key pulses 52are to be produced, whilst the said fly-back pulses are availablewithout the need for further means.

Thus the object aimed at is achieved.

FIGURE 7 shows a possible embodiment of the reactance circuit 56. Thisreactance circuit 50 consists of a discharge valve 52,, of which theanode is connected to the oscillator circuit 53, which determines thefrequency of the oscillator 48 (not shown in FIGURE 7). The dischargevalve 52 is connected in known manner as an inductive reactance valve bymeans of a blocking capacitor 54, a resistor 56 and a capacitor 57, sothat, when signals are fed to the control-grid of the valve 52, thereaotance thereof varies, and the oscillator 48 is readjusted. To thecontrol grid of the valve 52 are fed, via the conductor 58, the directvoltages emanating from the phase detector 49, which voltages aresmoothed by a filter 59 and provide the synchronization of theoscillator 48. Via the conductor 69, the separation resistor 61 and theparallel circuit 62 the correction signals from 51 are fed also to thecontrol-grid of the valve 52. The separation resistor 63 serves toseparate the correction signals from the signals of the phase detector49. The circuit 62 is tuned to the frequency of 4.5 mc./s. in order toavoid a reaction of the oscillator signal from 48 on the adder 14.

It should finally be noted that other voltages than added parabolicvoltages of lineand raster-frequency may be fed as correction signals tothe phase-shifting members 13 and 21. In the foregoing reference isalways made to the phase error due to the transit time of the secondaryelectrons. However, a second error may occur, i.e. due to turning of thepilot beam and the writing beam about each other during the deflection,mainly when they are at the remote corners of the screen. Also thisphase error may be compensated by introducing an additional phase shiftinto the signal supplied to the writing beam or the pilot beam, butowing to the deviating character of the last-mentioned phase error ascompared with that due to the transit time of the secondary electrons,other correction signals are required.

The nature of the phase error due to turning of the two beams about eachother is as follows. On the upper side of the screen this error is at amaximum, so that at the beginning of a line period it assumes a maximumvalue, it decreases gradually to zero and increases after half a lineperiod, however, with opposite sign. At the end of the line period amaximum phase error occurs, of which the value is the same but of whichthe sign is opposite that of the error at the beginning of the lineperiod.

The aforesaid phase errors occurring at the scanning of a line on theupper side of the screen decrease gradually in a vertical sense and aresubstantially equal to Zero in the centre of the screen throughout oneline period. Then they increase, however, with opposite sign. On thelower side of the screen similar maximum phase errors occur as on theupper side, but they have a phase shift of just 180.

As a correction signal use is therefore to be made of a sawtooth signalof a frequency equal to that of the horizontal deflection signal, whichsignal is modulated in the rhythm of the raster frequency. Thismodulation is to be carried out so that the sawtooth correction signalof line frequency has a maximum slope at the beginning of a rasterperiod, this slope becoming gradually less steep to attain the valuezero after half a raster period. In the second half of the raster periodthis slope becomes gradually steeper, but :with opposite polarity and atthe end of one raster period it is equal to that at the beginning, butthe polarity being opposite.

This may be carried out by means of a push-pull modulator, for examplethe know-n push-pull modulator consisting of an output transformer andan input transformer, the secondary winding of the input transformerbeing connected via four diodes to the primary winding of the outputtransformer.

The sawtooth signal of line frequency is to be fed to the primarywinding of the input transformer and the sawtooth signal of rasterfrequency between the central tappin-gs of the secondm and the primarywinding of the input transformer and the output transformerrespectively. The secondary winding of the output transformer of thispush-pull modulator is connected either to the phaseshifting member 13or to the phase-shitfting member 21.

Consequently, correction signals may be fed to the phase-shiftingmembers 13 and 21, which signals compensate not only the phase error dueto the transit-time effect of the secondary electrons but also that dueto the turning of the two beams about each other by feeding to thephase-shifting members not only the parabolic voltages but also theline-frequency sawtooth voltages modulated by the raster frequency.

What is claimed is:

l. A circuit for controlling a color television display tube of the typehaving a display screen with color stripes and index stripes, electrongun means for directing at least one electron beam toward said screen,and control electrode means for controlling said beam, and wherein saidindex stripes emit secondary electrons when struck by said beam forproviding an index signal, said circuit comprising a source ofdeflection signals for said display tube, means for integrating saiddeflection signals, a source of color video signals, a source ofreference oscillations, a source of local oscillations, means for mixingsaid local oscillations, local oscillations, reference oscillations andindex signal for application to said control electrode, an inductorhaving first and second serially-connected portions, means applying saidlocal oscillations to said indoctor, a series circuit of a resistor anda variable reactance device connected in parallel with said inductor,

said reactance device comprising an electron discharge device having atleast a control grid and an anode, means applying said integrateddeflection signals to said control grid whereby the oscillations at saidanode have the frequency of said local oscillations and a phasedependent upon said integrated signals, and means for connecting saidanode to said control electrode means for correcting for phase errors insaid index signal due to variations in transit time of said secondaryelectrons.

2. A circuit for controlling a color television display tube of the typehaving a display screen with color stripes and index stripes, electrongun means for directing at least one electron beam toward said screen,and control electrode means for controlling said beam, and wherein saidindex stripes emit secondary electrons when struck by said beam forproviding an index signal, said circuit comprising a source ofdeflection signals for said display tube, means for integrating saiddeflection signals, a source of color video signals, a source ofreference oscillations, a source of local oscillations, means forapplying said local oscillations to said control electrode, first,second and third mixer means, means for applying said local oscillationsand color signals to said first mixer means, phase-shifting means, meansfor applying said reference oscillations to said phase-shifting means,means for applying the outputs of said first mixer means andphase-shifting means to said second mixer means, means for applying theoutput of said second mixer means and said index signal to said thirdmixer means, and means applying the output of said third mixer means tosaid control electrode, said phase-shifting means comprising an inductorhaving first and second portions, means applying said referenceoscillations to said inductor, a series circuit of a resistor and avariable reactance device connected in parallel with said inductor, saidreactance device comprising an electron discharge device having at leasta control grid and an anode, means applying said integrated deflectionsignals to said control grid whereby oscillations at said anode have thefrequency of said reference oscillations and a phase dependent upon saidintegrated signals, and means for connecting said anode to said secondmixer means for correcting for phase errors in said index signal due tovariations in transit time of said secondary electrons.

3. A circuit for controlling a color television display tube of the typehaving a display screen with color stripes and index stripes, electrongun means for directing at least one electron beam toward said screen,and control electrode means for controlling said beam, and wherein saidindex stripes emit secondary electrons when struck by said beam forproviding an index signal, said circuit comprising a source ofdeflection signals for said display tube, means for integrating saiddeflection signals, a source of color video signals, a source ofreference oscillations, a source of local oscillations, a source ofburst signals, means for mixing said local oscillations, color signals,reference oscillations, and index signal for application to said controlelectrode, means for applying said local oscillations to said controlelectrode, and a control circuit for said source of referenceoscillations comprising reactance circuit means for varying the phase ofsaid reference oscillations, means for comparing said burst signals andreference oscillations to provide a control voltage for said reactancecircuit means, and gate means for applying said integrated deflectionsignals to said reactance circuit means only when burst signals are notpresent.

4. A circuit for controlling a color television display tube of the typehaving a display screen with color stripes and index stripes, electrongun means for directing at least one electron beam toward said screen,and control electrode means for controlling said beam, and wherein saidindex stripes emit secondary electrons when struck by said beam forproviding an index signal, said circuit comprising a source ofdeflection signals for said display tube, means for integrating saiddeflection signals, a source of color video signals, a source ofreference oscillations, a source of local oscillations, a source ofburst signals, means for mixing said local oscillations, color signalsand index signal for application to said control electrode means, meansfor mixing said local oscillations and reference oscillations forapplication to said control electrode means, and means for controllingsaid reference oscillations comprising reactance circuit means forvarying the phase of said reference oscillations, phase detector meansfor comparing said reference oscillations and burst signals to provide acontrol voltage for said reactance circuit means, and gate means forapplying said integrated deflection signals to said reactance circuitmeans only when said burst signals are not present.

5. A circuit for controlling a color television display tube of the typehaving a display screen with color stripes and index stripes, electrongun means for directing at least one electron beam toward said screen,and control electrode means for controlling said beam, and wherein saidindex stripes emit secondary electrons When struck by said beam forproviding an index signal, said circuit comprising a source ofdeflection signals for said display tube, means for integrating saiddeflection signals, a source of color video signals, a source ofreference oscillations, a source of burst signals, means for mixing saidcolor signals, reference oscillations and index signals, first gatemeans for alternately applying said index signals and said mixed colorand index signals and reference oscillations to said control electrodemeans, and means for controlling said reference oscillations comprisingreactance circuit means for varying the phase of said referenceoscillations, phase detector means for comparing said referenceoscillations and burst signals to provide a control voltage for saidreactance circuit means, and gate means for applying said integrateddeflection signals to said reactance circuit means only when said burstsignals are not present.

References Cited in the file of this patent UNITED STATES PATENTS2,648,722 Bradley Aug. 11, 1953 2,899,600 Bryan Aug. 11, 1959 2,990,446Moore June 27, 1961

5. A CIRCUIT FOR CONTROLLING A COLOR TELEVISION DISPLAY TUBE OF THE TYPEHAVING A DISPLAY SCREEN WITH COLOR STRIPES AND INDEX STRIPES, ELECTRONGUN MEANS FOR DIRECTING AT LEAST ONE ELECTRON BEAM TOWARD SAID SCREEN,AND CONTROL ELECTRODE MEANS FOR CONTROLLING SAID BEAM, AND WHEREIN SAIDINDEX STRIPES EMIT SECONDARY ELECTRONS WHEN STRUCK BY SAID BEAM FORPROVIDING AN INDEX SIGNAL, SAID CIRCUIT COMPRISING A SOURCE OFDEFLECTION SIGNALS FOR SAID DISPLAY TUBE, MEANS FOR INTEGRATING SAIDDEFLECTION SIGNALS, A SOURCE OF COLOR VIDEO SIGNALS, A SOURCE OFREFERENCE OSCILLATIONS, A SOURCE OF BURST SIGNALS, MEANS FOR MIXING SAIDCOLOR SIGNALS, REFERENCE OSCILLATIONS AND INDEX SIGNALS, FIRST GATEMEANS FOR ALTERNATELY APPLYING SAID INDEX SIGNALS AND SAID MIXED COLORAND INDEX SIGNALS AND REFERENCE OSCILLATIONS TO SAID CONTROL ELECTRODEMEANS, AND MEANS FOR CONTROLLING SAID REFERENCE OSCILLATIONS COMPRISINGREACTANCE CIRCUIT MEANS FOR VARYING THE PHASE OF SAID REFERENCEOSCILLATIONS, PHASE DETECTOR MEANS FOR COMPARING SAID REFERENCEOSCILLATIONS AND BURST SIGNALS TO PROVIDE A CONTROL VOLTAGE FOR SAIDREACTANCE CIRCUIT MEANS, AND GATE MEANS FOR APPLYING SAID INTEGRATEDDEFLECTION SIGNALS TO SAID REACTANCE CIRCUIT MEANS ONLY WHEN SAID BURSTSIGNALS ARE NOT PRESENT.